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The government has condemned the opposition, saying a rally would not remove the prime minister

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The sit-in is expected to continue throughout the weekend, although numbers are fewer than expected

Mr Khan, who heads the opposition Pakistan Tehreek-e-Insaaf party, said that the government was illegitimate, citing fraud and a failure to organise a probe into the elections.

He called for new elections, vowing to continue a sit-in until Mr Sharif resigns.

Police estimated the crowd was 60,000 strong.

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At the scene: M Ilyas Khan

There is a carnival atmosphere on Islamabad's two parallel roads where the two opposition leaders, Imran Khan and Tahirul Qadri, are addressing thousands of followers. Most of the crowd comprises young men, but there are also women and children.

Most protesters have been walking since Friday when the march started from Lahore. They braved last night's heavy downpours. Many went away during the night, but many more have come back to listen to the leaders lay down their demands.

Many participants said they wanted change and would hang on for as long as their leaders wanted. But Mr Qadri's marchers in January of last year, facing the absence of toilets and an outbreak of viral diseases, could only continue for three days before he signed a hurried deal with the then government and called off the protest.

Mr Khan's convoy of protesters was pelted with stones on the way, in the city of Gujranwala.

Officials from Mr Khan's party said his vehicle was also shot at but he was not hurt.

A senior leader of Mr Sharif's ruling Pakistan Muslim League party announced that the government was ready to investigate the allegations of fraud from last year's elections, but said that Mr Sharif would not be removed through a rally.

The government says the protesters are trying to derail democracy. The prime minister's spokesman Pervaiz Rashid condemned the "irresponsible behaviour and actions" of his opponents.

Mr Khan's and cleric Tahirul Qadri's supporters are angry about the sinking economy, growing militancy, and failure to deliver core services such as a steady electricity supply.